The invention relates to a voltage intermediate circuit converter having a 12-pulse input converter, a voltage intermediate circuit and a machine converter using three-point switching.
A converter of the type described above is disclosed in the German magazine xe2x80x9cetzxe2x80x9d, Issue 20, 1998, pages 10 to 12. This voltage intermediate circuit converter has, in its standard version, a 12-pulse diode rectifier, wherein the diode rectifier elements are each connected to a secondary winding of a three-winding transformer. On the DC-side, the diode rectifier elements are each linked to a capacitor in a voltage intermediate circuit, which has two capacitors which are connected electrically in series. Such an input converter is referred to as a diode front end (DFE). In most cases, such a diode front end satisfies the requirements for the mains system power factor and harmonic content. If mains feedback effects are subject to more stringent requirements, then a 24-pulse input converter is available.
A voltage intermediate circuit converter which has a self-commutated pulse-controlled converter as the input converter is described in the German magazine xe2x80x9cEngineering and automationxe2x80x9d, Issue 1-2, 1998, pages 8 and 9. Like the machine-side pulse-controlled converter, this input converter is in the form of a three-point pulse-controlled converter. The voltage intermediate circuit is formed by two capacitors which are electrically connected in series. This input converter circuit option is also referred to as an active front end (AFE). An active front end allows four-quadrant operation (driving and regenerative braking in both rotation directions). This active input converter not only allows a power factor of cos xcfx86=1 to be achieved, but also allows the wattless component of other loads to be compensated for in the mains system, at least as far as power margins are concerned. If the active front end is equipped with an input filter, virtually harmonic-free operation from the mains system is also possible.
A diode front end has the disadvantage that four-quadrant operation is not possible without further complexity. The additional complexity is that a break chopper is required for generator operation, by means of which the generated energy is converted into heat in a breaking resistance. The use of 12-pulse and 24-pulse diode front ends means that 5th, 7th, 11th and 13th harmonics, and 5th, 7th, 13th, 23rd and 25th harmonics, respectively, are suppressed. In the 24-pulse embodiment of the diode front end, the complexity on the input side is twice that of the 12-pulse embodiment of the diode front end, which means that it is not just the space requirement that increases.
An active front end has the disadvantage that the 5th, 7th, 11th and 13th, etc. harmonics occur, depending on the number of pulses, the amplitudes of which can at least be minimized by means of an optimized pulse pattern. Furthermore, the active front end is more complex than a diode front end owing to the number and configuration of the components. Since, in design terms, the active front end corresponds to the machine-side self-commutated pulse-controlled converter, a voltage intermediate circuit converter with an active front end occupies a larger amount of space than a voltage intermediate circuit converter with a 12-pulse diode front end.
The present invention is based on the object of specifying a voltage intermediate circuit converter having an input converter designed so that the harmonics which occur on the mains system side are kept as low as possible, and with minimal complexity.
Since the converter elements of the 12-pulse input converter are each self-commutated pulse-controlled converters, the advantages of a diode front end are combined with those of an active front end. This means that the harmonic currents of the 5th, 7th, 17th and 19th harmonics are suppressed on the mains system side of the voltage intermediate circuit converter without the optimized pulse patterns of the self-commutated pulse-controlled converters being optimized to these harmonics. Since the two converter elements are in the same operating state, their pulse patterns are the same. This optimized pulse pattern can now be optimized such that the amplitudes of the harmonic currents of the 11th, 13th, 25th, etc. harmonics are minimized.
A further advantage of this input converter according to the present invention is evident at very high voltages. The converters for standard medium voltages have two or more active converter devices connected in series for a voltage value above 3.3 kV. Since the input converter according to the invention has two identical self-commutated pulse-controlled converters, which are connected electrically in series, the number of converter elements connected in series is equal to or one less than the number of machine converters connected in series. With the standard medium voltage of 4.16 kV, the input converter of a voltage intermediate circuit converter according to the invention has precisely the same number of active converter devices as an input converter in the active front end configuration. Low blocking-capability semiconductor switches, which can be operated at a higher switching frequency or can be used at higher current levels, can be used as the active conductor devices, with precisely the same number connected in series. The design of the phase modules is both simple and space-saving, with the number of items connected in series being reduced by one.